man3/dl_iterate_phdr.3

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  24 .TH DL_ITERATE_PHDR 3 2007-05-18 "GNU" "Linux Programmer's Manual"
  25 .SH NAME
  26 dl_iterate_phdr \- walk through list of shared objects
  27 .SH SYNOPSIS
  28 .nf
  29 .B #define _GNU_SOURCE
  30 .B #include <link.h>
  31
  32 \fBint dl_iterate_phdr(\fP
  33           \fBint (*\fPcallback\fB) \
  34 (struct dl_phdr_info *\fPinfo\fB,\fP
  35                            \fBsize_t\fP size\fB, void *\fPdata\fB),\fP
  36           \fBvoid *\fPdata\fB);\fP
  37 .fi
  38 .SH DESCRIPTION
  39 The
  40 .BR dl_iterate_phdr ()
  41 function allows an application to inquire at run time to find
  42 out which shared objects it has loaded.
  43
  44 The
  45 .BR dl_iterate_phdr ()
  46 function walks through the list of an
  47 application's shared objects and calls the function
  48 .I callback
  49 once for each object,
  50 until either all shared objects have been processed or
  51 .I callback
  52 returns a non-zero value.
  53
  54 Each call to
  55 .I callback
  56 receives three arguments:
  57 .IR info ,
  58 which is a pointer to a structure containing information
  59 about the shared object;
  60 .IR size ,
  61 which is the size of the structure pointed to by
  62 .IR info ;
  63 and
  64 .IR data ,
  65 which is a copy of whatever value was passed by the calling
  66 program as the second argument (also named
  67 .IR data )
  68 in the call to
  69 .BR dl_iterate_phdr ().
  70
  71 The
  72 .I info
  73 argument is a structure of the following type:
  74
  75 .in +4n
  76 .nf
  77 struct dl_phdr_info {
  78     ElfW(Addr)        dlpi_addr;  /* Base address of object */
  79     const char       *dlpi_name;  /* (Null-terminated) name of
  80                                      object */
  81     const ElfW(Phdr) *dlpi_phdr;  /* Pointer to array of
  82                                      ELF program headers
  83                                      for this object */
  84     ElfW(Half)        dlpi_phnum; /* # of items in \fIdlpi_phdr\fP */
  85 };
  86 .fi
  87 .in
  88
  89 (The
  90 .IR ElfW ()
  91 macro definition turns its argument into the name of an ELF data
  92 type suitable for the hardware architecture.
  93 For example, on a 32-bit platform,
  94 ElfW(Addr) yields the data type name Elf32_Addr.
  95 Further information on these types can be found in the
  96 .IR <elf.h> " and " <link.h>
  97 header files.)
  98
  99 The
 100 .I dlpi_addr
 101 field indicates the base address of the shared object
 102 (i.e., the difference between the virtual memory address of
 103 the shared object and the offset of that object in the file
 104 from which it was loaded).
 105 The
 106 .I dlpi_name
 107 field is a null-terminated string giving the pathname
 108 from which the shared object was loaded.
 109
 110 To understand the meaning of the
 111 .I dlpi_phdr
 112 and
 113 .I dlpi_phnum
 114 fields, we need to be aware that an ELF shared object consists
 115 of a number of segments, each of which has a corresponding
 116 program header describing the segment.
 117 The
 118 .I dlpi_phdr
 119 field is a pointer to an array of the program headers for this
 120 shared object.
 121 The
 122 .I dlpi_phnum
 123 field indicates the size of this array.
 124
 125 These program headers are structures of the following form:
 126 .in +4n
 127 .nf
 128
 129 typedef struct {
 130     Elf32_Word  p_type;    /* Segment type */
 131     Elf32_Off   p_offset;  /* Segment file offset */
 132     Elf32_Addr  p_vaddr;   /* Segment virtual address */
 133     Elf32_Addr  p_paddr;   /* Segment physical address */
 134     Elf32_Word  p_filesz;  /* Segment size in file */
 135     Elf32_Word  p_memsz;   /* Segment size in memory */
 136     Elf32_Word  p_flags;   /* Segment flags */
 137     Elf32_Word  p_align;   /* Segment alignment */
 138 } Elf32_Phdr;
 139 .fi
 140 .in
 141
 142 Note that we can calculate the location of a particular program header,
 143 .IR x ,
 144 in virtual memory using the formula:
 145
 146 .nf
 147   addr == info\->dlpi_addr + info\->dlpi_phdr[x].p_vaddr;
 148 .fi
 149 .SH RETURN VALUE
 150 The
 151 .BR dl_iterate_phdr ()
 152 function returns whatever value was returned by the last call to
 153 .IR callback .
 154 .SH VERSIONS
 155 .BR dl_iterate_phdr ()
 156 has been supported in glibc since version 2.2.4.
 157 .SH "CONFORMING TO"
 158 The
 159 .BR dl_iterate_phdr ()
 160 function is Linux-specific and should be avoided in portable applications.
 161 .SH EXAMPLE
 162 The following program displays a list of pathnames of the
 163 shared objects it has loaded.
 164 For each shared object, the program lists the virtual addresses
 165 at which the object's ELF segments are loaded.
 166
 167 .nf
 168 #define _GNU_SOURCE
 169 #include <link.h>
 170 #include <stdlib.h>
 171 #include <stdio.h>
 172
 173 static int
 174 callback(struct dl_phdr_info *info, size_t size, void *data)
 175 {
 176     int j;
 177
 178     printf("name=%s (%d segments)\\n", info\->dlpi_name,
 179         info\->dlpi_phnum);
 180
 181     for (j = 0; j < info\->dlpi_phnum; j++)
 182          printf("\\t\\t header %2d: address=%10p\\n", j,
 183              (void *) (info\->dlpi_addr + info\->dlpi_phdr[j].p_vaddr));
 184     return 0;
 185 }
 186
 187 int
 188 main(int argc, char *argv[])
 189 {
 190     dl_iterate_phdr(callback, NULL);
 191
 192     exit(EXIT_SUCCESS);
 193 }
 194 .fi
 195 .SH "SEE ALSO"
 196 .BR ldd (1),
 197 .BR objdump (1),
 198 .BR readelf (1),
 199 .BR dlopen (3),
 200 .BR feature_test_macros (7),
 201 .BR ld.so (8),
 202 and the
 203 .I "Executable and Linking Format Specification"
 204 available at various locations online.